When your part comes out of the CNC process, different levels of finishes are available. Here is an overview of the options of post-processing treatment to improve the surface finish and resistance.
Right out of the CNC machine, the piece have visible marks of tool of around 3.2 RA µm which are visible on the surface. Those marks can be reduced to 1.6µm, 0.8µm or 0.4µm if the finish requirements are higher. As machined is the most popular finish for functionnal parts as it represents about ¾ of total finishes. It is the most economical finish and helps to get the best tolerance. The second place goes to anodize finish: not only the parts is more beautiful but it also helps to get chemical and abrasion resistance.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. It increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does The oldest anodizing process uses chromic acid. But due to the safety regulations regarding air quality control, type II with sulfuric acid is preferred. A variety of different colors can be applied, and for the Type III, a wear resistant finish layer is added on top of the corrosion resistance of Type II.
Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys.
Anodizing changes the microscopic texture of the surface and the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances.
Bead blasting is the process of removing surface deposits by applying fine glass beads at a high pressure without damaging the surface. Bead blasting is used in creating a uniform surface finish on machined parts. It gives a matte finish with light texture.
Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in coating and is then cured under heat to allow it to flow and form a "skin". The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Because powder coating does not have a liquid carrier, it can produce thicker coatings than conventional liquid coatings without running or sagging, and powder coating produces minimal appearance differences between horizontally coated surfaces and vertically coated surfaces. Powder coat finishes are avaiable in a large range of colors.